** spoiler alert ** It constantly amazes me the gulf there is between Card's obnoxious and forcefully expressed social opinions and the humanitarian voice that speaks through many of his novels, but never more so than in this instance.

In the not-so-far future, the earth seems to be recovering from the onslaught of the 20th and 21st centuries. The human population has stabilized at a smallish fraction of the current level. Employees of the organization called Pastwatch spend their time scanning history using a device that can passively observe the past . . . and, they begin to suspect, perhaps not entirely passively, because it seems the occasional sensitive can detect the "presence" of the observers. One research project is to pinpoint those moments in history whose consequences led to the devastation of the planet and the human species's current sorry shape. The crucial moment appears to be Christopher Columbus's decision, in the wake of his near-miraculous survival after a shipwreck, to sail westward across the Atlantic in search of new lands to exploit and new souls to save for Christ. The Pastwatchers are appalled when they witness the instant of his making that decision: he received a visitation from two human figures and a dove -- Father, Son and Holy Ghost, in the iconography of the time -- who told him the westward voyage should be his holy duty. The Pastwatchers recognize these figures must be time travellers from an alternate future, a future so bad that its denizens have resorted to altering the past in order to erase it and of course, in the process, themselves.

[Why should that be an inevitable consequence? In the physics of time expressed in this book, there are no alternative realities: always only the one. This is because the quanta of time -- "moments" -- are quite discrete from each other. "When the machine was introduced into our history, from that point forward a new infinite set of moments completely replaced the old infinite set of moments. There were no spare leftover moment-locations for the old moments to hang around in." (p194) And the fact that the time device was created in a future that, the instant it used the device, ceased ever to have existed is no paradox because, while it seems to us that causality is timelike, in fact causality is independent of time and its functionings unamenable to rational analysis (rather like the Jungian concept of synchronicity): in particular, "Causality can be recursive but time cannot." (p193).]

So some event in the other history happened that would have been forestalled by Columbus sailing the ocean blue, as he did in our own past. What could it have been? A young Pastwatcher builds a near-watertight case that it was the discovery, not of the New World by the Old, but of the Old World by the New -- the conquest of Europe by the Tlaxcalans, who brought with them the hideous practice of mass human torture-sacrifice that was rife among all the South American cultures of the era.

Around this time in our future, time travel is developed, and the Pastwatchers are obviously highly interested by this field of technology so close to the one they're using. Also, it occurs to them that, just like their counterparts, they could perhaps alter history to lead to a happier outcome. This notion is spurred by the discovery that the earth is not in fact recovering, as people had thought: any recovery will be centuries or millennia in the future, by which time the human species will be long gone. Again they focus on Columbus. If they could, by scuttling his ships, make it impossible for him immediately to return to Europe after his New World landfall, and if they could play upon the Christian sensibilities of the man such as to deflect his mind off gold and slavery toward converting the local civilizations to good liberal values . . . So they send back three volunteers to the 15th century, thereby erasing everyone else after Columbus's time (us included) from reality. Can those three pull off the task?

This book is extraordinarily slow to get itself off the ground -- about two-thirds of the text is occupied by the setting-up (alternating between the 15th century and the Pastwatchers' present) preparatory to the actual time travel -- and the characterization of some of the major movers and shakers manages to achieve the feat of seeming both laboured and perfunctory at the same time. Much of that setting-up is in itself fascinating (there's a really neat Atlantis/Noah hypothesis!), and it was only a few times that a sense of "oh, for gawd's sake get on with it" swept over me. The writing is up to Card's usual high and elegant standard (although near the end there are some signs of apparent haste). Overall, I'd recommend this to friends as worth their time.